1. Field of the Invention
The present general inventive concept relates to an image forming apparatus to control a density of a toner supplied by a developing unit and a control method thereof, and more particularly, to an image forming apparatus which reduces consumption of a toner used to detect a density of a toner and improves halftone realization to thereby control a density of a supplied toner, and a control method thereof.
2. Description of the Related Art
Generally, a bi-component type image forming apparatus forms a toner image on a print medium with a developer having a toner forming an image and a carrier carrying the toner by a magnetic force. The image forming apparatus should be required to control a density of a toner, i.e., a mixture ratio between a toner and a carrier included in the developer to thereby supply the toner consistently for a printing job.
A conventional toner density control method counts a number of pixels in an image to be printed and controls driving time of a toner supply driving source according to the counted number of pixels to supply the toner. The conventional toner density control method estimates an amount of the consumed toner based on the number of pixels. If there is an error between the estimated toner consumption amount and the actually-supplied toner amount, the density of the toner increases or decreases by a long-term toner supplying operation.
To solve the foregoing problem, another conventional toner density control method uses a patch pattern to detect a density of a toner.
The conventional toner density control method forms a patch pattern on a photosensitive body or on an intermediate transfer body, and detects a density of the patch pattern and changes in the density to thereby adjust the toner supply amount. In this case, the patch pattern should be formed frequently to detect the density thereof and to maintain the density of an output image consistently. However, if the image pattern is frequently formed to detect the density, the toner consumption increases and the toner amount used for the image forming operation decreases. Thus, a patch pattern-forming cycle should be optimized.
The relation between the density of the toner supplied by the developing unit and the density of an image formed on the print medium will be described. If the density of the toner is higher, the density of the image is higher, too. If the density of the toner is lower, the density of the image is accordingly lower. If a determination is made according to the detection result of the patch pattern that the density of the image is low, the toner supply amount based on the number of pixels is adjusted so that more toner is supplied. If a determination is made that the density of the image is high, the toner supply amount based on the number of pixels is reduced.
However, even though the toner amount is adjusted, there is a difference between the actual toner consumption amount and the toner supply amount. Thus, if the printing job is performed under such a circumstance, the density of the toner is gradually higher or lower than a target density to make the density of the image irregular.
According to the conventional toner amount control method, the toner supply amount may be adjusted by using lookup table data. That is, the conventional toner amount control method adjusts the toner supply amount by changing the range of values used among the values in the lookup table, according to the density change of the detected patch pattern. Meanwhile, there is a time delay between a timing of determining that the density of the patch pattern is lower or higher than the target density, and a timing of getting the toner to the target density.
To prevent such a problem, the image forming operation is suspended momentarily after detecting the density of the patch pattern. If the density of the patch pattern is lower than the target density, the toner is forcibly supplied. By contrast, if the density of the patch pattern is higher than the target density, the toner is forcibly consumed. In this case, the image forming operation is suspended for a long time, thereby taking more time to form an image. Also, as the toner is forcibly consumed, the toner is wasted.
Another conventional toner density control method uses a developing bias to maintain a consistent density of an image and to prevent drastic changes in the density.
The conventional toner density control method detects a density of a toner with a patch pattern and adjusts the density of an image by controlling a developing bias. The conventional method addresses problems such as deterioration of a developer due to environmental changes or lifespan with only the developing bias. Thus, the variable ranges of the developing bias should be wide. If the developing bias is set too high, however, a carrier is developed to a photosensitive body together with the toner and the image may be deteriorated. If the developing bias is set too low, realization of halftone is lowered.